1. Field of the Invention
This invention relates to an apparatus and a method for induction heating of pieces or blanks of electrically conducting and non-magnetic material.
2. Description of the Related Art
Induction heating is used in force pressing in order to soften up metal bolts or billets before they are being pressed into profiles. For non-magnetic materials which are good electrical conductors, such as aluminium, copper or brass, conventional induction heating has an efficiency of only 55-60%. In such conventional heating processes a bolt or billet is axially placed within a coil. Alternating current is applied to the coil, so that an axial alternating magnetic field is produced. Consequently counter-current is induced in the bolt or billet to counteract the magnetic field. In this case the induced current heats the press bolt or billet. The problem here is that the current in the coil causes losses that are of the same size as in the bolt or billet, which causes the efficiency to become low.
Induction heating devices can also be equipped with superconducting coils for alternating current, cf. Norwegian Patent No. 308.980. Superconductors, however, produce losses when they are exposed to an alternating magnetic field. A problem that can occur here is that the heat from the AC losses in the superconductors must be cooled away (at approximately 50-90 Kelvin), and the cooling system, that is needed, is expensive.
Recently, there have appeared possibilities for formation of a static or DC magnetic field without energy losses. Superconductors can, under DC conditions, conduct electric current practically without losses, and strong permanent magnets have become available at a reasonable price. In the solution proposed here the superconductors are substantially exposed only to a static or DC magnetic field, and therefore a substantially smaller cooling system is required, which is also cheaper than the one used in the induction heating apparatus employed in Norwegian Patent No. 308.980.